Binding of Nitriles and Isonitriles to V(III) and Mo(III) Complexes: Ligand vs Metal Controlled Mechanism.

The synthesis and structures of nitrile complexes of V(N[Bu]Ar), (Ar = 3,5-MeCH), are described. Thermochemical and kinetic data for their formation were determined by variable temperature Fourier transform infrared (FTIR), calorimetry, and stopped-flow techniques. The extent of back-bonding from metal to coordinated nitrile indicates that electron donation from the metal to the nitrile plays a less prominent role for than for the related complex Mo(N[Bu]Ar), . Kinetic studies reveal similar rate constants for nitrile binding to , but the activation parameters depend critically on the nature of R in RCN. Activation enthalpies range from 2.9 to 7.2 kcal·mol, and activation entropies from -9 to -28 cal·mol·K in an opposing manner. Density functional theory (DFT) calculations provide a plausible explanation supporting the formation of a π-stacking interaction between a pendant arene of the metal anilide of and the arene substituent on the incoming nitrile in favorable cases. Data for ligand binding to do not exhibit this range of activation parameters and are clustered in a small area centered at Δ = 5.0 kcal·mol and Δ = -26 cal·mol·K. Computational studies are in agreement with the experimental data and indicate a stronger dependence on electronic factors associated with the change in spin state upon ligand binding to .